The invention relates to adsorbent materials used in pressure swing adsorption (PSA) processes. More particularly, this invention relates to PSA processes for the production of high purity oxygen (e.g. oxygen having a purity of 90-95 vol. %O2). More particularly, the invention relates to the selection of adsorbent materials for use in PSA processes. More particularly, the invention relates to the adsorbent materials which have been selected on the basis of related combinations of their intrinsic adsorption rate and adsorption equilibrium properties.
There has been significant development of the various PSA, VSA and VPSA methods for air separation over the past thirty years, with major advances occurring during the last decade. Commercialization of these processes and continued extension of the production range can be attributed primarily to improvements in the adsorbents and process cycles, with-advances in adsorber design contributing to a lesser degree. Conventional adsorbents for PSA O2 are N2-selective at equilibrium and include 13X, CaA, CaX, and mixtures of CaA and CaX, for example. Advanced adsorbents have improved equilibrium properties such as high xcex94N2 loading (e.g. working capacity), high nitrogen/oxygen selectivity and high capacity. Highly exchanged lithium molecular sieve adsorbents, as illustrated by Chao in U.S. Pat. No. 4,859,217, typify such advanced adsorbents for O2 production.
Improving process efficiency and reducing the cost of the light component product can be accomplished by decreasing the amount of adsorbent required and by increasing the product recovery. The former is generally expressed in terms of bed size factor (BSF) in pounds adsorbent/TPDO (ton per day of contained O2), while the latter is simply the fraction of light component in the feed that is captured as product.
Improvement in adsorbents and reduction in cycle time are two primary methods of reducing BSF. While shorter cycles lead to shorter beds and higher adsorbent utilization, product recovery generally suffers unless adsorption rate is increased. This phenomenon can be ideally characterized in terms of the size of the mass transfer zone (MTZ), i.e. the mass transfer zone becomes an increasing fraction of the adsorbent bed as the bed depth decreases. Since the adsorbent utilization with respect to the heavy component is much lower in the MTZ than in the equilibrium zone, working capacity (e.g. xcex94N2 loading) declines as this fraction increases.
Attempts have been made to correlate the adsorbent materials properties more explicitly with their performance in process cycles. The well-known increases in mass transfer rates available with smaller adsorbent particles have been included in cycle improvements: (1) with dual, separated adsorption layers by Sircar (U.S. Pat. No. 5,071,449), (2) with short cycle times and sequenced steps by Hirooka et. al. (U.S. Pat. No. 5,122,164), and (3) in beds of different particle sizes with cross flow of gas by Hay et. al. (U.S. Pat. No. 5,176,721). Gaffney et. al. have used cycle studies to identify ranges of practical value for combinations of equilibrium materials properties: (1) isothermal working capacity and amount of inert diluent (U.S. Pat. No. 5,258,060) and (2) isothermal working capacity with selectivity (U.S. Pat. No. 5,266,102).
These examples of prior art typify two approaches in the correlation of equilibrium-selective adsorbent materials properties with process performance. In the first approach, the adsorbent composition. is taken as fixed, then mass transport is considered separately and accommodated by adjustments in particle size and bed design. In the second approach, the adsorbent composition is varied to alter equilibrium properties, then mass transport is measured, if it is considered at all.
Finally, Moreau et al. (U.S. Pat. No. 5,672,195) has suggested higher porosity in zeolites to achieve improved O2 yield and throughput in PSA air separation. A preferred porosity range of 0.38 to 0.60 is claimed in conjunction with a minimum rate coefficient. Moreau et al. state that commercially available zeolites are not suitable for their invention since porosity is lower than 0.36. Moreau et al. fail to address the significant offsetting effects of high porosity.
It is therefore an object of the invention to provide for a PSA process having improved performance characteristics.
It is a further object of the invention to improve PSA process performance via the use of enhanced adsorbent materials.
It is a further object of the invention to improve PSA process performance through the use of adsorbent materials having improved intrinsic sorption rate characteristics.
It is a further object of the invention to improve PSA performance through the use of adsorbent materials having intrinsic sorption rates that are correlated with the adsorbent material""s equilibrium characteristics.
It is a further object of the invention to provide a process for selecting adsorbents for PSA processes.
The invention relates to the use of adsorbent materials that have been selected on the basis of preferred and related combinations of their intrinsic adsorption rate and adsorption equilibrium properties.